Thermoelectric generation technology is a technology for directly converting thermal energy into electrical energy using the Seebeck effect, in which an electromotive force is generated in proportion to a temperature difference created between opposite ends of a substance. This technology is being used practically for example, for a remote area power supply, an outer space power supply, and a military power supply.
A conventional thermoelectric device has a configuration that is referred to as a “π-type structure” in which thermoelectric materials of a p-type semiconductor and an n-type semiconductor, having carriers of opposite signs, are combined together to be connected to each other thermally in parallel and electrically in series.
Generally, the performance of a thermoelectric material used for a thermoelectric device is evaluated by a figure of merit Z, or a figure of merit ZT that is obtained by multiplying a figure of merit Z by absolute temperature to be non-dimensionalized. The figure of merit ZT can be expressed as ZT=S2T/ρκ, where S is a Seebeck coefficient, ρ is electrical resistivity, and κ is thermal conductivity, of a substance. The figure S2/ρ, which is indicated by the Seebeck coefficient S and electrical resistivity ρ, is a value referred to as a power factor that is used as a measure for determining the quality of the power generation performance of the thermoelectric material and thermoelectric conversion device under a constant temperature difference.
Bi2Te3 based materials such as Bi2-aSbaTe3 (0≦a≦2) that currently is used practically as the thermoelectric material have relatively high thermoelectric properties with a ZT of about 1 and a power factor of 40 to 50 μW/cmK2 under the present conditions. However, even the thermoelectric device having a π-type structure containing the Bi2Te3 based material used therein cannot be said to have a sufficiently high power generation performance for being used practically in a wider range of applications.
Meanwhile, as a thermoelectric device having a structure other than the π-type structure, a thermoelectric device long has been proposed that takes advantage of the anisotropy of thermoelectric properties of natural or artificially-produced layered structures (see, for example, Non-Patent Literature 1). Patent Literature 1 describes a thermoelectric device that takes advantage of the anisotropy of thermoelectric properties of a layered structure composed of metal and Bi, which is a thermoelectric material. The thermoelectric device described in Patent Literature 1 has a power factor that surpasses by far that of the thermoelectric device having a π-type structure containing a material Bi or the Bi2Te3 based material used therein, by suitably selecting the ratio in thickness between metal and Bi or the inclination angle in the layered direction.    [Prior Art Literature]    [Patent Literature]    [Patent Literature 1] JP 4078392 B    [Non-Patent Literature]    [Non-Patent Literature 1] A. A. Snarskii, P. Bulat, “THERMOELECTRICS HANDBOOK”, Chapter 45, CRC Press (2006)